This invention relates to methods for depositing a layer of fibers from a gaseous suspension of fibers onto a moving foraminous forming surface. More specifically, this invention sets forth improvements in the methods of achieving high speed production of nonwoven fibrous webs.
In the production of nonwoven webs, the prior art has generally disclosed systems which form an air suspension of fibers and direct the suspended fibers toward a moving foraminous forming surface upon which the fibers from the suspension are condensed to form the nonwoven web. Various apparatus exist for generating the air suspension of fibers; for example, fibers can be produced by a lickerin, a hammermill, or by other apparatus known in the art. The fibers can be dispersed in a gaseous medium by a variety of methods, and conveyed to a forming surface in an air stream operating generally as a pressure flow system, a vacuum system or a closed loop system. Applicant has found that critical process limitations exist in the production of such air laid nonwoven webs, in that in a given system as the speed of the forming surface increases, the uniformity of the formed web tends to decrease. It has been found that particularly at commercial speeds above about 550 feet per minute, fiber lay-down on the moving forming surface tends to become uneven in the machine direction, with the deposited webs exhibiting an upper surface having an undulated, wave-like or ripple effect extending in the cross-machine direction, and with the webs exhibiting corresponding variations in thickness and in basis weight. This rippling effect becomes more pronounced as the speed of the forming surface increases, with the spacing between the thicker portions of the web becoming greater and with the web becoming more varied along its surface as to basis weight. This rippling effect worsens with increased speed of the forming surface, and eventually renders the webs commercially unacceptable at some higher speed. This invention is directed at significantly alleviating the detrimental ripple effect. Applicant has determined that there exists a critical relationship between the velocity of the fibers relative to the velocity of the moving forming surface as the fibers are being deposited onto the forming surface. Applicant has further determined that the detrimental ripple effect can be controlled by controlling the ratio of the tangential velocity of the fibers relative to the moving forming surface, as compared to the velocity of the fibers normal to the moving surface.
Some prior art has been concerned with obtaining higher production speed in the production of air laid webs. In U.S. Pat. No. 4,004,323, issued to Gotchel et al., for example, it is taught that speeds in excess of 200 feet per minute can be achieved by inclining a duct, which carries a stream of fibers, at an angle to a forming surface. The patent teaches a range of incidence angles of the duct relative to the forming surface of from about 10.degree. to about 30.degree.. Gotchel fails, however, to teach the high speed formation limitation termed herein the rippling effect, nor does Gotchel contain any teaching as to the criticality of the velocity ratios as between the fibers and the forming surface in alleviating the detrimental rippling effect. Indeed, the present invention teaches that there are combinations of fiber stream velocities and forming surface velocities for which ducts aligned as recommended in Gotchel will produce unacceptable webs with pronounced ripples.